1. Field of the Invention
The present invention relates to a beam converting apparatus suitable for beam shaping used in optical disk and other optical pickups.
2. Description of the Prior Art
A fundamental construction of a beam shaping optical system in an optical pickup is, hitherto, as shown in FIG. 1. That is, as the light source for optical pickup, generally, a semiconductor laser element 11 is used. Its far-field pattern, or the light intensity distribution of the beam section is approximately elliptical. The luminous flux emitted from the semiconductor laser element 11 is converted into a parallel luminous flux by a collimator lens 12, and is applied to a beam shaping optical system 15 in which the light intensity distribution of the beam section is shaped into a circular form, and the luminous flux is focused on the signal plane of an optical disk 14 by an objective lens 13 to record or reproduce the signal. In the semiconductor laser element 11, structurally, the emission angle is wide in the direction vertical, and narrow in the direction parallel, to the junction plane of the semiconductor laser chip. Expressing each emission angle as .theta.10 and .theta.11, the relation is generally .theta.10:.theta.11=approx. 3:1. That is, l1 in FIG. 1 is the luminous flux in the .theta.10 direction, and l2 is the luminous flux in the .theta.11 direction, and by expanding the luminous flux l2 by the beam shaping optical system 15, the light intensity distribution of the beam section is shaped into a circular form.
In the prior art having the structure as shown in FIG. 2 (e.g. The Japanese Patent Publication Sho. No. 61-53775), the optical system 15 is bent at an acute angle by the shaping prism 16, and the optical pickup is increased in size, while the shape of the optical block, that is, the housing is complicated and the cost is high. Furthermore, when attempted to thin the optical pickup, a triangular mirror or a plate mirror known as a riser mirror is placed behind the shaping prism 16 so as to deflect the light by 90 degrees, and this riser mirror was also a cause of increasing the cost.